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Stock-Based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2018
Stock-Based Compensation [Abstract] |
Weighted average assumptions used to determine fair value for stock awards | The fair value for stock awards was estimated at the date of grant using the Black-Scholes option valuation model with the following weighted average assumptions for the years ended December 31, 2018, 2017 and 2016:
| Year Ended December 31,
| 2018 |  | 2017 |  | 2016
Expected life (in years) |  | 6.2 |  |  | 6.4 |  |  | 6.5
Interest rate |  | 2.50% |  |  | 2.04% |  |  | 1.72%
Volatility |  | 30.33% |  |  | 36.61% |  |  | 36.75%
Expected dividend yield |  | 0.26% |  |  | 0.26% |  |  | 0.54%
Summary of stock option activity | The following table summarizes stock option activity for the year ended December 31, 2018:
|  | Awards |  |  | Weighted Average Exercise Price per Share |  | Weighted Average Remaining Contractual Life (Years) |  | Aggregate Intrinsic Value (Millions) |
Awards outstanding at December 31, 2017 |  |  | 996,839 |  |  | $ | 48.21 |  |  |  |  |
Granted |  |  | 191,147 |  |  |  | 76.09 |  |  |  |  |
Exercised |  |  | (98,945 | ) |  |  | 33.83 |  |  |  |  |
Canceled |  |  | (34,782 | ) |  |  | 65.74 |  |  |  |  |
Awards outstanding at December 31, 2018 |  |  | 1,054,259 |  |  | $ | 54.04 |  |  |  | 6.10 |  |  | $ | 7.8 |
Awards exercisable at December 31, 2018 |  |  | 688,652 |  |  | $ | 46.17 |  |  |  | 4.90 |  |  | $ | 6.9 |
Non-vested stock option activity | Non-vested stock option activity for the year ended December 31, 2018 is as follows:
|  | Awards |  |  | Weighted Average Grant Date Fair Value per Share |
Nonvested awards outstanding at December 31, 2017 |  |  | 400,393 |  |  | $ | 58.94 |
Granted |  |  | 191,147 |  |  |  | 76.09 |
Vested |  |  | (195,055 | ) |  |  | 56.17 |
Canceled |  |  | (30,878 | ) |  |  | 65.13 |
Nonvested awards outstanding at December 31, 2018 |  |  | 365,607 |  |  | $ | 68.86 |
Summary of restricted stock activity | The following table summarizes the restricted stock activity for the Plan:
|  | Awards |  |  | Weighted Average Grant Date Fair Value per Share |
Unvested balance at December 31, 2017 |  |  | 180,110 |  |  | $ | 58.57 |
Granted |  |  | 69,361 |  |  |  | 76.26 |
Vested |  |  | (59,649 | ) |  |  | 56.44 |
Canceled |  |  | (55,244 | ) |  |  | 58.57 |
Unvested balance at December 31, 2018 |  |  | 134,578 |  |  | $ | 68.64 |

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- Definition Tabular disclosure for stock option plans. Includes, but is not limited to, outstanding awards at beginning and end of year, grants, exercises, forfeitures, and weighted-average grant date fair value. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of stock options, including, but not limited to: (a) expected term of share options and similar instruments, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition Disclosure of the number and weighted-average grant date fair value for restricted stock and restricted stock units that were outstanding at the beginning and end of the year, and the number of restricted stock and restricted stock units that were granted, vested, or forfeited during the year. + References No definition available. + Details
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